Acoustic wave devices using a piezoelectric thin film resonator are used as filters of wireless devices, for example. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. A region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region. Examples of the acoustic wave devices using the piezoelectric thin film resonator include a filter and a duplexer. The piezoelectric film, the lower electrode, and the upper electrode of the piezoelectric thin film resonator usually have negative temperature coefficients of an elastic constant. Thus, the resonance frequency of the piezoelectric thin film resonator shifts to a low frequency side as temperature increases. As described above, in the acoustic wave device, the resonance frequency, the anti-resonance frequency, and the passband change in accordance with temperature.
There has been known a piezoelectric thin film resonator that uses an insulating film such as a silicon oxide film having a temperature coefficient opposite to those of the piezoelectric film, the lower electrode, and the upper electrode as a temperature compensation film to prevent the frequency from changing in accordance with the temperature (e.g. Japanese Patent Application Publication Nos. 58-137317 (Patent Document 1) and 60-16010 (Patent Document 2)).
There has been known a piezoelectric thin film resonator that provides conductive films on the upper and lower surfaces of the insulating film to cause short circuits or a piezoelectric thin film resonator that embeds a temperature compensation film in the lower electrode or the upper electrode to prevent the decrease of an electromechanical coupling coefficient due to the use of the temperature compensation film (Japanese Patent Application Publication No. 60-16010 (Patent Document 2), U.S. Patent Application Publication No. 2011/0266925 (Patent Document 3), and U.S. Pat. No. 6,420,820 (Patent Document 4)).
It has been known that resonance characteristics degrade when the upper electrode is thinner in the peripheral portion of the resonance region than in the center portion (e.g. FIG. 8 in Japanese Patent Application Publication No. 2006-109472 (Patent Document 5)).
When the temperature compensation film is embedded in the lower electrode or the upper electrode as disclosed in Patent Documents 3 and 4, the lower electrode or the upper electrode becomes thinner in the peripheral portion of the resonance region than in the center portion. Thus, resonance characteristics degrade as disclosed in FIG. 8 of Patent Document 5.